Bromate ions, reactions

The bromate-ferroin reaction has a quadratic autocatalytic sequence, but in this case the induction period is detennined primarily by the time required for the concentration of the hiliibitor bromide ion to fall to a critical low value tlirough the reactions... 

The existence of chaotic oscillations has been documented in a variety of chemical systems. Some of tire earliest observations of chemical chaos have been on biochemical systems like tire peroxidase-oxidase reaction [12] and on tire well known Belousov-Zhabotinskii (BZ) [13] reaction. The BZ reaction is tire Ce-ion-catalyzed oxidation of citric or malonic acid by bromate ion. Early investigations of the BZ reaction used tire teclmiques of dynamical systems tlieory outlined above to document tire existence of chaos in tliis reaction. Apparent chaos in tire BZ reaction was found by Hudson et a] [14] aiid tire data were analysed by Tomita and Tsuda [15] using a return-map metliod. Chaos was confinned in tire BZ reaction carried out in a CSTR by Roux et a] [16, E7] and by Hudson and... 

The equation for the reaction between iodide and bromate ions in acidic solution is... 

A voltaic cell consists of two half-cells. One of the half-cells contains a platinum electrode surrounded by chromium(III) and dichromate ions. The other half-cell contains a platinum electrode surrounded by bromate ions and liquid bromine. Assume that the cell reaction, which produces a positive voltage, involves both chromium(III) and bromate ions. The cell is at 25°C. Information for the bromate reduction half reaction is as follows ... 

This subject has been reviewed by Noyes and Field,8 who give reference to the original formulation as well as a more explicit treatment. The presentation here will be given not in general terms but by means of one striking example, the oxidation of malonic acid by bromate ions catalyzed by cerium(IV). It is called the Belousov-Zhabotinsky (or BZ) reaction, after its discoverers.9 The stoichiometry of the reaction with excess malonic acid is... 

A synthetically useful reaction has been reported between alkaline bromine water and dimethyl sulphoxide118, the product being the perbromosulphone (equation 36). A kinetic study of the oxidation of dimethyl sulphoxide by bromate ions, catalysed by ruthenium(III) salts, has also been published but no yield data are available119. 

The products of the reaction between bromide ions and permanganate ions, Mn04, in basic aqueous solution are solid manganese(IV) oxide, MnO>, and bromate ions. Balance the net ionic equation for the reaction. 

The separation in Part a is obvious. Oxidation numbers confirm the other assignments. In Part b, recall that whereas the oxygen atom in H2 O has an oxidation number of-2, the oxygen atoms in H2 O2 have oxidation number -1. In Part c, the oxidation number of the bromine atoms in Br02 is -I3. Among the products, bromine is -H5 in Br03 and -1 in Br. Thus, bromite ions are oxidized to bromate ions in one half-reaction and reduced to bromide ions in the other half-reaction. 

An acidic bromate solution can oxidize various organic compounds and the reaction is catalyzed by species like cerous and manganous ions that can generate 1-equivalent oxidants with quite positive reduction potential. Belousov (1959) first observed oscillations in Celv]/[Cem] during Ce (III) catalysed oxidation of citric acid by bromate ion. Zhabotinskii made extensive studies of both temporal and spatial oscillations and also demonstrated that instead of Ce (III), weak 1- equivalent reductants like Mn(II) and Fe (II) can also be used. The reaction is called Belousov-Zhabotinskii reaction. This reaction, most studied and best understood, can be represented as... 

In the presence of an oxidant, e.g., chlorate or bromate ions, the electrode reaction is transposed into an adsorption coupled regenerative catalytic mechanism. Figure 2.85 depicts the dependence of the azobenzene net peak current with the concentration of the chlorate ions used as an oxidant. Different curves in Fig. 2.85 correspond to different adsorption strength of the redox couple that is controlled by the content of acetonitrile in the aqueous electrolyte. In most of the cases, parabolic curves have been obtained, in agreement with the theoretically predicted effect for the surface catalytic reaction shown in Fig. 2.81. In a medium containing 50% (v/v) acetonitrile (curve 5 in Fig. 2.85) the current dramatically increases, confirming that moderate adsorption provides the best conditions for analytical application. 

Reaction 12.21 illustrates the trend toward lower electronegativity (less oxidizing power) as one descends a periodic group. The bromine vapor is trapped in aqueous Na2C03 (in effect, a mild source of alkali) as bromide and bromate ions... 

Numerous versions of the Belousov-Zhabotinsky system differ by chemical compounds used. The typical reaction involves oxidation of some organic compound by bromate ion (BrOj ) occurring in acid medium with metal catalyst (Ce3+, Mn2+, as well as complexes of Fe2+, Ru2+). As an example, a particular reaction [4] could be mentioned, where an organic reductor is malonic acid CH2(COOH)2 and Ce3+ ions serve as a catalyst. In this reaction a solution changes periodically its colour due to oscillations in Ce3+ concentration. Generally speaking, the reaction consists of two stages. At the first one metal is oxidized... 

The uncatalysed Belousov-Zhabotinsky (B-Z) reaction between malonic acid and acid bromate proceeds by two parallel mechanisms. In one reaction channel the first molecular products are glyoxalic acid and carbon dioxide, whereas in the other channel mesoxalic acid is the first molecular intermediate. The initial reaction for both pathways, for which mechanisms have been suggested, showed first-order dependence on malonic acid and bromate ion.166 The dependence of the maximal rate of the oxidation of hemin with acid bromate has the form v = [hemin]0-8 [Br03 ] [H+]12. Bromate radical, Br02, rather than elemental bromine, is said to play the crucial role. A mechanism has been suggested taking into account the bromate chemistry in B-Z reactions and appropriate steps for hemin. Based on the proposed mechanism, model calculations have been carried out. The results of computation agree with the main experimental features of the reaction.167... 

The reaction above is reversible, for if the colorless basic solution is acidified, the elementary halogen is regenerated. Hypohalites will not survive in warm solution, but undergo further self-oxidation or dis-mutation. Typically, the OBr ion is converted to BrOi" and Br it follows, therefore, that basic hydrolysis of bromine results ultimately in bromide and bromate ions in about a 5 1 ratio ... 

Example 26 Bromate ions can be reduced by iodide in acid medium. Express the reaction equation. 

The reaction is first-order bromide ion, first-order bromate ion, and second-order hydrogen ion. What is the rate law and overall order If the proton concentration quadruples, what is the effect on the rate ... 

The reaction between bromate ions and bromide ions in acidic aqueous solution is given by the following equation ... 

The oxidation of [W(CN)8] by bromate ions in perchloric acid solution 101) was found to be partially autocatal3dic due to the following reactions ... 

The rate of protonation of bromate ion has been inferred from the kinetic currents observed in the polarographic reduction . The rate coefficients increase slowly with pH (1.0x10 l.mole . sec at pH 1.45, 8.0x10 at 4.55). lodate exchanges oxygen with water relatively easily, and the reaction shows... 

Direct interference from the bromate-HCl reaction should be too slow to be significant. Although extensive formation of a complex e.g. Br02SCN) followed by rate-determining reaction with thiocyanate ion and an alternative (chloride ) path, viz. 

Bromate itself reacts only slowly with reductants the reactive substance is Br2 formed according to (20-9), BrCl, or a mixture of BrCl and CI2 formed in solutions containing chloride. The electrode potential is controlled by the halide concentration, and the reaction is slow in the absence of both chloride and bromide. In (20-9) the rate of reaction is proportional to bromate ion concentration, bromide ion concentration, and hydrogen ion concentration squared. ... 

In the BZ reaction, malonic acid is oxidized in an acidic medium by bromate ions, with or without a catalyst (usually cerous or ferrous ions). It has been known since the 1950s that this reaction can exhibit limit-cycle oscillations, as discussed in Section 8,3. By the 1970s, it became natural to inquire whether the BZ reaction could also become chaotic under appropriate conditions. Chemical chaos was first reported by Schmitz, Graziani, and Hudson (1977), but their results left room for skepticism—some chemists suspected that the observed complex dynamics might be due instead to uncontrolled fluctuations in experimental control parameters. What was needed was some demonstration that the dynamics obeyed the newly emerging laws of chaos. 

One of the best known and studied oscillating reactions is the Belousov-Zhabotinsky reaction. This is the oxidation of an organic compound, like malonic acid, in a sulfuric acid solution by the bromate ion BrOJ. The reaction is catalysed by a redox catalyst like Ce(III)/Ce(IV), Mn(II)/Mn(III) or Fe(phen)3 "/Fe(phen)3". The simplified overall chemical reaction is... 

When bromide ion, Br- is virtually absent, bromate ion, Br03 reacts with cerium(III) and malonic acid, such that Process B results from four reactions. 

Example 14.3. The Belousov-Zhabotinsky reaction [22,27-29], The reaction is an oxidation of malonic acid by bromate ion in sulfuric acid, catalyzed by a Ce(III)/Ce(IV) redox couple. Many variations with other organic acids and transition-metal ions are possible [22] (Belousov used citric acid, and manganese, ruthenium, or iron can replace cerium). The color of the solution alternates between clear [Ce(III)] and pale yellow [Ce(IV)], and more dramatically between red and blue if ferroin is added as indicator. 

Despite the importance of the chlorite-iodide systems in the development of nonlinear chemical dynamics in the 1980s, the Belousov-Zhabotinsky(BZ) reaction remains as the most intensively studied nonlinear chemical system, and one displaying a surprising variety of behavior. Oscillations here were discovered by Belousov (1951) but largely unnoticed until the works of Zhabotinsky (1964). Extensive description of the reaction and its behavior can be found in Tyson (1985), Murray (1993), Scott (1991), or Epstein and Pojman (1998). There are several versions of the reaction, but the most common involves the oxidation of malonic acid by bromate ions BrOj in acid medium and catalyzed by cerium, which during the reaction oscillates between the Ce3+ and the Ce4+ state. Another possibility is to use as catalyst iron (Fe2+ and Fe3+). The essentials of the mechanisms were elucidated by Field et al. (1972), and lead to the three-species model known as the Oregonator (Field and Noyes, 1974). In this... 

Zhabotinsky A M, Buchholtz F, Kiyatin A B and Epstein I R 1993 Oscillations and waves in metal-ion catalysed bromate oscillating reaction in highly oxidised states J. Phys. Chem. 97 7578-84... 

In its classical form the BZ reaction consists of a one-electron redox catalyst, an organic substrate that can be easily brominated and oxidized, and a bromate ion in fonn of NaBrOj or KBrCh all dissolved in sulfuric or nitric acid. As catalysts, mostly Ce(III)/Ce(IV) salts, Mn(II) salts and ferroin are used. As organic substrate normally malonic acid (HOOC-CH2-COOH, MA) is used, instead of Belousov s original citric acid he was looking for an inorganic analog of Krebbs cycle. 

For reaction (c) which represents the reaction between bromate ion (Br03 ) and bromide ion (Br-) in acidic aqueous solution, the experimental rate equation is / = / R[Br03-][Br-][H+]2... 

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